Reinforced concrete structures sometimes suffer deterioration because the reinforcing steel eventually corrodes. This is often caused by chloride contamination or carbonation of the concrete. Sacrificial anodes are used to inhibit the corrosion of steel in concrete. Sacrificial anode assemblies, embedded within cavities formed in concrete, typically comprise a sacrificial metal that is less noble than steel (i.e., electrochemically more negative than steel) such as zinc, an activator for maintaining an activity of the sacrificial metal element, a backfill for accommodating the products of the sacrificial metal dissolution and a ductile elongate metal conductor for interconnecting the sacrificial metal element with the steel or to a power supply.
In some cases-typically the case when a consumed activator is used, the backfill and the activator will be assembled with the sacrificial metal and the conductor in a factory and this assembly will then be installed on site by embedding the assembly in a cementitious mortar in a cavity formed within the concrete. In cavities formed as the result of corrosion damage, the sacrificial anode assembly will be “tied” or otherwise electrically coupled or connected to the steel before substantially filling the cavity with a concrete repair material in order to restore the exterior profile and properties of the concrete substantially back to its original state. In other cases, an anode cavity (for example, a drilled hole) may be mechanically formed for the purpose of installing a sacrificial anode assembly and, in this case, the anode cavity will be substantially filled with the assembly (see Repair Application Procedure 8 published by the American Concrete Institute at www.concrete.org/generall/RAP-8.pdf).
More recent developments include assembling the components of a sacrificial anode assembly within an anode cavity. For example, a sacrificial metal element may be embedded directly in a backfill located in the anode cavity, as disclosed in U.S. Pat. No. 8,002,964. The anode cavity may also open into a larger cavity formed as the result of corrosion damage to protect the steel in the adjacent undamaged concrete.
It is to be appreciated that the embedded sacrificial anodes, for reinforced concrete structures, need to be activated if they are to provide the desired protection to the reinforcing steel. Activators for sacrificial anodes may be described as a consumed activator or a catalytic activator. An example of an activator that is consumed is hydroxyl ions. The hydroxyl ions react with the sacrificial metal element to produce a soluble species. It is to be appreciated that with this form of activation, the functional life of the sacrificial anode assembly depends on the quantity of the activator present. Such activators are generally pre-assembled as an anode-backfill assembly. It is to be appreciated that such activators may also be aggressive to the parent concrete by, for example, causing an adverse alkali silica reaction to occur within the parent concrete.
Examples of catalytic activators—which include halide ions and sulphate ions—are found in, for example, U.S. Pat. No. 7,749,362. The halide ions render passive films on a sacrificial metal element unstable. One problem associated with this form of activation is that such activators also tend to be aggressive with respect to the reinforcing steel and this can cause corrosion problems.
Another problem with sacrificial anode assemblies containing activators is that they may, when located in contact with the air, gradually deteriorate because an active metal in contact with an activator normally suffers from atmospheric corrosion. As a result, this phenomena tends to limit the shelf life of such a sacrificial anode assembly.
Sacrificial anode assemblies are sometimes used in combination with a power supply to deliver an impressed current treatment, for example, as disclosed in U.S. Pat. No. 7,909,982. This arrangement can be also used to draw chloride ions, present in the concrete, to the sacrificial metal element to activate the anode assembly. In this case, an activator(s) may not be required and an increased shelf life of the assembly can be achieved.
When a sacrificial anode assembly is assembled from components and placed within a cavity formed in an underside or a downwardly facing opening in a reinforced concrete element another problem occurs, namely, the anode may fall out of the cavity due to the effect of gravity until such time as the backfill sufficiently hardens or a concrete repair material covers the assembly to prevent inadvertent removal thereof.